Bio 111 Ch 27 - Archaea and Bacteria

Bio 111: Chapter 27 - Archaea and Bacteria

Learning Objectives

  • Explain what prokaryotes have in common.

  • Explain why Archaea and Bacteria have been placed in separate domains and which shares a more recent common ancestor with Eukarya.

  • Describe the typical cellular structure of prokaryotes, including:

    • Range of sizes of prokaryotic cells.

    • Nature of genetic material.

    • Organelles and cell wall structure.

  • Explain the distinction between Gram-positive and Gram-negative bacteria.

  • Using examples, explain the term "extremophile".

  • Assess whether all Archaea are extremophiles.

  • Describe three different prokaryote species and compare them based on cell structure, metabolism, and ecological niche.

Prokaryotic Facts

  • Prokaryotes are the most abundant organisms on Earth:

    • Comprise half of Earth’s biomass.

    • A pinch of soil can contain up to 2 billion prokaryotes.

  • They occupy every conceivable habitat:

    • Include extremes like deep Arctic ice, inside rocks, boiling and acidic hydrothermal vents, and floating in the clouds.

  • They are considered the earliest life forms.

Modes of Nutrition in Prokaryotes

  • Heterotrophs: Require organic molecules for nourishment.

  • Autotrophs: Capable of producing their own organic molecules.

  • Heteroautotrophs: Can utilize both organic and inorganic sources.

Oxygen Requirements

  • Aerobes: Require oxygen to survive.

  • Anaerobes: Are poisoned by oxygen.

  • Facultative Anaerobes: Can live with or without oxygen.

  • Significance in ecosystems:

    • Some are decomposers and nitrogen fixers, crucial for ecological balance.

Reproduction in Prokaryotes

  • Prokaryotes reproduce asexually through binary fission, producing two genetically identical cells.

  • This rapid cell division allows for fast population growth, especially in colonies.

Gene Transfer Mechanisms in Prokaryotes

  • Various mechanisms facilitate DNA transfer between prokaryotic cells:

    • Transformation: Bacteria uptake genetic fragments from the environment.

    • Transduction: Viruses (bacteriophages) transmit genes between bacteria.

    • Conjugation: Direct transfer of DNA through a mating bridge between cells.

    • Plasmid transmission: Sharing small circular DNA molecules (plasmids) among bacteria.

Biofilms in Prokaryotes

  • Prokaryotes often form biofilms, organized colonies of one or several species attached to a surface like rocks or living tissues.

Taxonomy of Prokaryotes

  • Historical Classification:

    • Initially classified as either plants or animals.

    • Later, five kingdoms emerged (Monera, Protista, Plantae, Fungi, Animalia).

  • Current Classification:

    • The three-domain system:

    • Bacteria

    • Archaea

    • Eukarya

    • The Monera kingdom was abandoned due to its inclusion of both Archaea and Bacteria.

  • Characteristics of prokaryotes:

    • They are unicellular, lack a nucleus and organelles, and are generally smaller and simpler than eukaryotic cells.

Horizontal Gene Transfer

  • Horizontal gene transfer involves gene movement from one species to another, enhancing genetic diversity.

  • Vertical gene transfer refers to the transfer of genetic material from parent to offspring.

  • Studies suggest that at least 17% of the genes in E. coli originated from other bacteria, and about 80% of prokaryotic genes have undergone horizontal transfer at some point in their evolution.

Distinctions Between Bacteria and Archaea

Characteristics of Domains Bacteria, Archaea, and Eukarya

Feature

Bacteria

Archaea

Eukarya

Chromosomes

Usually circular

Circular

Usually linear

Nucleosome structure

No

No

Yes

Cell division method

Binary fission

Binary fission

Mitosis/meiosis

Introns in genes

Rarely

Rarely

Common

Ribosome size

70S

70S

80S

Initiator tRNA

Formylmethionine

Methionine

Methionine

Operons

Yes

Yes

No

Capping of mRNA

No

No

Yes

RNA polymerases

One

Several

Three

Membrane lipids

Ester-linked

Ether-linked

Ester-linked

Cell compartmentalization

No

No

Yes

Eons and Fossil Evidence

  • Timeline of Life's History:

    • 3.8-3.5 billion years ago: Prokaryotic cells first appeared.

    • 3.5 billion years ago: Fossils of primitive cyanobacteria documented.

    • 1.8 billion years ago: Eukaryotic cells emerged.

    • 1.5 billion years ago: Multicellular eukaryotes began to appear.

Relationships Between Bacteria and Other Species

  • Bacteria have co-evolved with every other species:

    • Each species has its unique microbiome.

    • Many ecological relationships are established for mutual benefit, minimizing harm.

  • Some bacteria are pathogenic, while others play beneficial roles.

Helpful Bacteria

  • Probiotic bacteria convert milk into yogurt through lactic acid fermentation and help restore gut microbiomes.

  • Decomposers are vital in recycling nutrients, particularly in sewage treatment.

  • Some heterotrophs aid in bioremediation, removing pollutants from the environment.

Harmful Bacteria

  • Pathogens: Microorganisms causing diseases in hosts by injecting harmful proteins or DNA (e.g., Salmonella, Yersinia pestis, Bordetella pertussis).

Basic Bacterial Structure

  • Unique Structures:

    • Capsule: A sticky outer coat.

    • Plasmids: Small rings of DNA.

    • Cytoplasm: Cell interior.

    • Cell wall: Rigid outer layer.

    • Plasma membrane: Regulates passage.

    • Pili: Short projections for attachment.

    • Ribosomes: Organelle that builds proteins.

    • Nucleoid: Coiled DNA without a membrane.

    • Flagellum: Aids in movement.

More Complex Structures in Prokaryotes

  • Thylakoids: Membrane folds involved in photosynthesis, increasing surface area for chlorophyll.

  • Magnetosomes: Organelles that enable direction finding in low oxygen environments, acting as organic compasses.

  • Gas vesicles: Buoy photosynthetic organisms to lighted water surfaces, creating scum.

Domain Bacteria Overview

  • Comprises approximately 50 bacterial phyla, with many structural and metabolic features still unknown.

  • While most bacteria prefer moderate conditions, many are extremophiles and engage in symbiotic relationships with eukaryotes (e.g., nitrogen fixation with plants, ruminant microbiome).

Examples of Phyla and Their Characteristics

  • Cyanobacteria: Photosynthetic bacteria that produce oxygen.

  • Chlamydiae: Minuscule obligate intracellular parasites.

  • Spirochaetes: Corkscrew-shaped motile bacteria, including pathogens like Treponema pallidum.

  • Actinobacteria: Notable antibiotic producers.

  • Proteobacteria: Diverse group including pathogens and species important in agriculture and biotechnology.

Cyanobacteria

  • Unique for their blue-green pigmentation and capability to perform oxygenic photosynthesis, contributing to eukaryotic plastids (chloroplasts).

  • Can generate toxic algal blooms in nutrient-rich waters, posing dangers to health.

Extremophiles

  • Many Archaea are extremophiles, thriving in extreme conditions, including:

    • Halophiles (high salt environments).

    • Hyperthermophiles (high temperatures, e.g., Methanopyrus grows at 98°C).

    • Acidophiles (acidic environments, e.g., Sulfolobus at pH 3).

Summary of Archaea Features

  • Archaea possess unique membrane lipids (ether-bonded) allowing resilience in extreme conditions.

  • Some Archaea exist in moderate environments, including human guts and ocean surfaces, highlighting their ecological versatility.

  • Research on Archaea is ongoing, with many features still unidentified due to difficulties in cultivation.